Engine control systems employ exhaust gas recirculation (EGR) mechanisms to regulate exhaust emissions and improve fuel economy. EGR mechanisms may include an EGR system which recirculates a portion of the exhaust gas from the exhaust passage to the intake passage via an EGR passage. The EGR systems employ a delta pressure (DP) sensor across an orifice located downstream of an EGR valve in the EGR passage to provide an estimate of EGR mass flow. The estimated EGR mass flow is then utilized to determine a degree of spark advance.
However, at certain engine operating conditions, such as at high load conditions, and/or when an manifold absolute pressure (MAP) is greater than a threshold pressure, a differential pressure across the orifice is modulated due to pulsating flow of the exhaust. Therefore, the DP sensor may output a higher voltage due to the root mean square value of the exhaust pulsations. In other words, the exhaust pulsation may cause the DP sensor to output a higher voltage than actual. As a result, EGR mass flow may be estimated to be higher than actual flow during the high load conditions. Since spark advance is based on the estimated EGR mass flow, typically one degree of spark advance for each percent of EGR estimated, for example, an overestimation of EGR mass may lead to potential spark knock (due to over-advanced spark timing). As a result, it may be necessary to retard the spark timing to reduce knock, which may lead to reduced fuel economy and performance.
The inventors herein have recognized the above-mentioned issues. Accordingly, in one example, some of the above issues may be at least partially addressed by a method for an engine, comprising: estimating an exhaust gas recirculation (EGR) mass flow based on a differential pressure sensor output when an engine load is below a threshold; estimating the EGR mass flow based on an intake carbon dioxide sensor output when the engine load is above the threshold and independent of the differential pressure sensor output; and adjusting a spark timing based on the estimated EGR mass flow. In this way, more accurate EGR flow estimations may be performed across various load conditions. Consequently, more accurate spark advance may be scheduled, which reduces the chances of spark knock.
As one example, during certain engine operating conditions, such as when an engine load is above a threshold load and/or when a MAP is above a threshold pressure, the EGR system may be operated in an open loop control mode. In the open loop control mode, the EGR mass flow is estimated independent of DP sensor output but rather based on feed forward mapped intake carbon dioxide data based on engine speed and load; and a degree of spark advance is scheduled based on the EGR mass flow estimated based on intake carbon dioxide values. Further, during the open loop mode, an EGR valve is not controlled based on DP sensor output but rather maintained in a fully open position or in a nearly fully open position that is based on threshold load.
During engine operating conditions below the threshold, the EGR system may be operated in a closed loop control mode. In the closed loop control mode, the EGR mass flow is estimated based on DP sensor output, and the degree of spark advance is scheduled based on the DP sensor based EGR mass flow estimation. Further, during the closed loop control mode, the EGR valve is controlled based on DP sensor output. For example, the EGR valve is adjusted based on an error between an actual DP sensor output and a desired DP sensor output.
In this way, by switching between open loop and closed loop control of the EGR system, more accurate EGR flow estimations may be performed. Consequently, more accurate spark advance may be scheduled, which may lead to reduced spark knock. As a result, unwarranted spark retard may be reduced, resulting in improved fuel economy and performance. Thus, by utilizing open loop control and closed loop control of the EGR system based on load and intake manifold pressure, the technical effect of more accurate EGR flow estimation, more accurate spark advance, and reduced spark knock may be achieved, and hence fuel economy may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.